Transformations of phenylacetylene and tert-butylacetylene in the presence of copper and copper oxide (Cusub2/subO, CuO) nanopowders prepared by gas-phase condensation of copper in argon were studied. The reaction of phenylacetylene with copper oxide nanopowders having different phase compositions in the absence of a solvent at room temperature resulted in oxidative condensation of phenylacetylene and complex formation of the condensation product. The complex undergoes decomposition by the action of acids, bases, and compounds capable of forming complexes. According to the X-ray diffraction data, one of the products is a new “parquet” modification of diphenyldiacetylene. Under analogous conditions, tert-butylacetylene gave rise to a complex mixture of products among which di- tert-butyldiacetylene was identified by gas chromatography-mass spectrometry. No copper complexes with the tert-butylacetylene condensation products were detected.